Lithium ion batteries (LIB) were conceived and developed in Japan by Asahi Kasei Co. and first commercialized by Sony Co. in 1991, and soon this novel energy storage technology had drawn tremendous attention from academic and industry communities alike. With many extensive research and engineering efforts conducted, lithium-ion batteries have become the most popular power source for portable electronic devices, such as, but not limited to, cell phones, mp3 players, and laptop computers. In addition, applications for use in the hybrid vehicle (HEV) and plug-in hybrid electric vehicle (PHEV) are being actively investigated. However, the improvements in battery performance must also be mindful of safety issues. When more energy or capacity is stored in the cell, the system becomes more thermodynamically unstable. For secondary lithium-ion batteries, dangerous results could be triggered under certain situations such as shortage, elevated temperature and overcharge.
Overcharge is a battery condition where electrical flow is forced through a cell after the capacity of the cell has been already been reached. Overcharge is one of the more common factors that could lead to serious safety issues in lithium-ion batteries. The phenomenon is most likely to occur during the charging of a battery pack. Due to the manufacturing processes, there will always be a weakest cell with the lowest capacity in one battery pack. During charging, the weak cell will always reach its full capacity before the other cells and without triggering the voltage monitor of the charger because the voltage of the full pack is still within the normal range. But the weak cell is none-the-less in a state of overcharge. As a result, extra electricity will build up on the surface of the electrodes instead of being stored, thereby dramatically increasing the potential of the cathode. As the charging is continued, the potential will go beyond the electrochemical window of the electrolyte and cause various reactions of the electrolyte. For example, oxidation of the electrolyte may occur and thereby trigger other reactions. The cell may end up in a thermal runaway, or even result in an explosion.